link to lecture transcript - UT-H GSBS Medical Physics Class Site

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Here’s an example of how we can estimate the electronic attenuation coefficient. We want to determine the electronic attenuation coefficient for a 0.8 MeV photon in a free electron gas. How do we determine this? First of all, the maximum energy of the electrons can be estimated from the graph or calculated from a formula to be 0.606 MeV. 28
Now we also look at the graph and see that dσ/dE is approximately constant with a value of 40 10 -30 m 2 /e MeV. To a first approximation, let’s forget about the Compton edge; we will just approximate the integration by assuming that dσ/dE is constant. To do the integration, we multiply dσ/dE by the energy interval, 0.606 MeV, and find that σ is the value of 24 10 -30 m 2 /e. Now if we look at the number in Johns & Cunningham we find it’s 23.5 10 -30 m 2 /e. So simply by assuming dσ/dE to be constant, we get a very good estimate of the total cross section. 29
Page 1 and 2: Today, we continue our talk about C
Page 3 and 4: We previously derived an equation f
Page 5 and 6: So σ 0 is the classical cross sect
Page 7 and 8: Let’s look at this graph. Notice
Page 9 and 10: What about the dependence of the Co
Page 11 and 12: The energy transferred to the elect
Page 13 and 14: If we plot the energy transfer coef
Page 15 and 16: The energy transfer coefficient has
Page 17 and 18: What about the scatter coefficient?
Page 19 and 20: Using this equation we can now plot
Page 21 and 22: This probability is represented by
Page 23 and 24: This figure is a plot taken from Jo
Page 25 and 26: We can always calculate what the ma
Page 27: Remember dσ/dE is relatively const
Page 31 and 32: First, let’s recall that the atte
Page 33 and 34: What we now want to find out is how
Page 35 and 36: So far, our approximations have bee
Page 37 and 38: These values were tabulated by Hubb
Page 39 and 40: So what is the effect of binding en
Page 41 and 42: At higher energies there is no diff
Page 43 and 44: Note that this graph is for very lo
Page 45: The fraction of energy transferred

link to lecture transcript - UT-H GSBS Medical Physics Class Site

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